On 8/4/2015 2:30 AM, Jeff wrote:

But there is a difference. With the resistor load the current flowing
from the inner lead of the coax *must* (by Kirchoff's 1st law) balance
the current flowing on the shield inner surface. So there is no current
remaining to flow on the shield outer surface. No diagram needed.

For the case of the dipole, the load is no longer balanced with the
shield outer surface connected to one side and there is no requirement
for the current in the two arms to be equal. So in this case current
flows on the shield outer surface in an amount inversely proportional to
the impedance seen by the current in the antenna element and the shield.


No, the dipole is still balanced, just as it would be if it were fed by
twin wire balanced feeder. The power will ALL be applied to the dipole
and, if perfectly matched, will ALL be radiated (or lost as heat).

The only current that will flow on the coax outer is due to the induced
currents from the radiated field, or that caused if the dipole is not
truly balanced.


If the dipole is truly balanced (and the coax is perpendicular to the
antenna), how does current get induced into the coax? My understanding
is that perpendicular wires have little current induction due to this
orientation.

Try a NEC simulation of a dipole in free space with say 5m of feeder
attached (wire that simulates the outer for NEC purposes) and look at
the differences in current flow in these 3 scenarios:


Is EZNEC sufficient for this?


1. Coax outer connected to one side of the dipole.

2. Coax outer NOT connected to the dipole outer by a very small
distance. (NEC cfeeder still connected)

3. Coax outer connected to one side of the dipole and an inductance
simulating a choke (balun if you wish to call it such) close to the dipole.

You will find very little difference in the currents flowing in the coax
outer in all 3 cases.


Free space is hard to come by for us amateurs. In your model, can you
attach the end opposite the antenna to ground and repeat the test?


The pattern will be distorted by the currents induced in the coax outer,
and there will be other small differences due to the small lack of
symmetry in the model. What you wont see is a huge difference in the
current in the outer when it is directly connected to one element of the
dipole.

Jeff


I don't remember that the possible pattern changes are in debate. But,
my memory is flawed and the thread is long.